In order to operate efficiently, airbreathing hypersonic vehicles generally rely on at least two propulsion system types to complete their missions: one to propel the vehicle at relatively low speeds (Mach 0 to 3-4), and one to take over at higher speeds (Mach 3-4 to Mach 7-9 for hydrocarbon-fueled accelerator and cruise vehicles, and up to Mach 10-12 for hydrogen-fueled cruisers). The low-speed propulsion system is typically a turbine engine, designed to survive the thermal stresses of high-Mach operation and supply adequate thrust over the required speed range. High-speed thrust may be provided by a dual-mode ram/scramjet.
Integration of these propulsion systems on a hypersonic vehicle may be enhanced by a common Multi-Role Air Induction System (MRAIS) to supply the needs of both propulsion system types, creating a so-called “turbine-base combined cycle” (TBCC) propulsion system. Requirements for an MRAIS include supplying the required amount of air with adequate pressure recovery and sufficient operability margin for each propulsion system independently, and also during propulsion system transition from low-speed to high-speed operating mode. MRAIS efficient operation and smooth mode transition rely on a well-designed, highly integrated system of inlet variable geometry and bleed.
Prior art hypersonic inlet systems typically include variable geometry systems that are used to redirect and compress the incoming airflow during various portions of the vehicle's flight regime. Known hypersonic inlet systems include, for example, those systems disclosed in U.S. Pat. No. 5,054,288 issued to Salemann, and U.S. Pat. No. 5,337,975 issued to Peinemann. Typically, prior art TBCC inlets have relied upon a variable planar (or two-dimensional 2D) geometry integrated into an over/under arrangement, with the turbine flowpath being above the ramjet/scramjet flowpath, and having the turbine inlets external to, and forward of, the ramjet/scramjet inlet, while sharing a common external forebody. Typically, planar variable geometry features (e.g., flat flaps with effective sealing) have not been effectively integrated with inlets which are defined by axisymmetric flowfields. Axisymmetric flowfield inlets may offer benefits, including more efficient compression in converging (i.e., inward turning) flows, than 2-D flowfields which can have stronger shock waves and greater losses. Thus, although such prior art hypersonic inlet systems may provide desirable results, there is room for improvement.